In the Muller-Rochow direct synthesis, methyl chloride is reacted with silicon in the presence of a copper catalyst and suitable promoters to form methylchlorosilanes. In this process, both the highest possible productivity (amount of silanes formed per unit time and amount of silicon used) and the highest selectivity, based on the target product dimethyldichlorosilane, is demanded. Dimethyldichlorosilane is required, for example, for preparing linear polysiloxanes.
Despite the great economic importance of the direct synthesis, its scientific background has still not been completely studied. According to Lieske et al. in SILICONE FOR CHEMICAL INDUSTRY, Geiranger-Norway, Jun. 16-18, 1992, owing to the participation of three solids in the reaction, that is to say silicon, catalyst and promoters, the reproducibility of the experiments is frequently poor. In practice, different batches of the direct synthesis, despite identical material parameters and reaction parameters, proceed with variable results.
The direct synthesis can be carried out batchwise or continuously, although in industrial production only the continuous variant is employed. The continuous direct synthesis is carried out in fluidized-bed reactors in which methyl chloride is used simultaneously as fluidizing medium and reactant. The silicon required is ground in advance to give a powder of particle size from 20 to 700 .mu.m and mixed with copper catalyst and promoters to form the catalyst composition.
Before the continuous direct synthesis production campaign, there is provided a "reactor heat-up phase", in which the catalyst composition is heated to reaction temperature. A continuous direct synthesis production campaign begins with the "induction phase". At the start of the induction phase, methyl chloride is introduced into the heated catalyst composition. This is followed by the "start phase" in which the crude silane formation is initiated. The reaction initially proceeds at low selectivity and reactivity. The stable "production phase" is then reached. The production campaign ends when methyl chloride is no longer introduced into the catalyst composition.
During the continuous operation of a reactor in a production campaign, after a substantially stable production phase, the production rate, based on methylchlorosilanes, and the selectivity, based on the target product dimethyldichlorosilane, both decrease. Therefore, the production campaign must be terminated after a certain time. As a result, a production campaign therefore usually lasts from only a few days to several weeks.
The reactor, after termination of a production campaign, is emptied, refilled with silicon, copper catalyst and promoters, and again brought to reaction conditions. It can be seen from the foregoing, that increasing the production rate and prolonging the duration of the production campaign, while retaining the same selectivity, increases the economic efficiency of the direct synthesis.
Activating the catalyst composition before the reaction with methyl chloride by a preliminary reaction with HCl is disclosed, for example, by U.S. Pat. No. 4,864,044. In the examples there, a process is described in which silicon, copper catalyst with or without tin promoter, but in the absence of zinc promoters, can be activated by HCl at approximately 325.degree. C. The disadvantages of this type of activation are that zinc or zinc compounds can be added only after the activation, since zinc forms readily sublimable zinc chloride with HCl under the specified reaction conditions and can thus be removed from the catalyst composition during the activation; a separate reactor is required for the activation and the reaction products of the activation were, in particular, trichlorosilane and tetrachlorosilane, are unwanted by-products of the methylchlorosilane synthesis; at least from 1 to 2% of the silicon raw material used is consumed by the activation; and a relatively high activation temperature is required.